(a) Field of the Invention
The present invention relates to a method for preparing super absorbent polymer. More particularly, the present invention relates to a method for preparing super absorbent polymer comprising two polymerization steps.
(b) Description of the Related Art
Super Absorbent Polymer (SAP) is synthetic polymer material that can absorb moisture of about 500 to 1000 times of the self weight, and is differently named as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) and the like according to development companies. The super absorbent polymer began to be commercialized as sanitary goods, and currently, it is widely used as a soil water retention agent for horticulture, water stop material for civil engineering and construction, a sheet for raising seeding, a freshness maintaining agent in the field of food distribution, and fomentation material and the like, in addition to hygienic goods such as a disposable diaper for children.
The super absorbent polymer can be generally prepared in a powder type product by polymerizing monomers for polymer and drying and pulverizing it.
In the process of preparing super absorbent polymer, the step of polymerizing monomers is an important step for determining the physical properties of polymer. As the polymerization method, reverse phase suspension polymerization, thermal polymerization and photopolymerization and the like are known. Among them, as the photopolymerization method, a method of putting a monomer composition for polymer on a belt and irradiating from the top to polymerize the monomer composition is known.
However, according to the above polymerization method, the amount of irradiation may not be uniform according to the depth of the monomer composition, and thus, polymerization degree may become non-uniform according to the depth. For example, in case a monomer composition of a predetermined thickness is put on a belt, on the upper part of the monomer composition located closely to light source, polymerization progresses relatively excessively due to the large amount of light irradiation, while on the lower part of the monomer composition located far from the light source, the amount of light irradiation is insufficient and thus polymerization may not be completely conducted and non-polymerized components may be remained. In this case, the physical properties of super absorbent polymer may be degraded. To overcome this, quantity of light or light irradiation time may be varied according to the location of the monomer composition solution, which makes the process complicated and increases process time, thus lowering productivity.